1. Field of the Invention
The present invention relates to a high-strength heat-resisting ferritic steel pipe or tube, more particularly, to a heat-resisting ferritic steel pipe or tube containing chromium, the pipe or tube having improved high temperature creep characteristics and excellent weldability and toughness.
2. Description of the Related Art
In recent years, in the field of thermal power plant, plant sizes are increasing and the operating temperatures and pressures are rising. When selecting steel pipe or tube (hereinafter in this section, collectively referred to as steel tube) for use at elevated temperatures exceeding 550.degree. C., inevitably high grade austenitic steel tubes, such as 18-8 stainless steel tubes are used instead of 21/4 Cr-1Mo ferritic steel tubes, from the viewpoint of oxidation resistance and high temperature strength.
As the grade of steel used becomes higher, i.e., from low alloy steel tube to stainless steel tube, or further, to super alloy tube, both tube and boiler construction costs are increased. This has led to the use of a super critical pressure boiler having an increased operating pressure, to improve boiler efficiency.
A steel tube that will fill the gap between 21/4 Cr-1Mo steel tubes and austenitic stainless steel tubes has been desired for many years. However, steel tubes with intermediate contents of Cr, i.e., 9Cr, 12Cr, etc., have an impaired weldability corresponding to an increase in the strength as compared with 21/4 Cr-1Mo steel tube. These steel tubes cannot be practically used because the impaired weldability considerably lowers the efficiency of boiler fabrication work.
Under these circumstances, research has been made by present inventors and others into the development of novel steel tubes having an improved weldability and a creep rupture strength superior to those of conventional tubes.
However, a further elevation of the steam temperatures utilized and frequent run/stop operations of the boiler caused by fluctuations in the demands for electric power are anticipated, and thus a reduced plant wall thickness, i.e., a further improved creep rupture strength, is desired in order to, e.g., mitigate thermal stress.
On the other hand, although it is disclosed in Japanese Examined Patent Publication (Kokoku) No. 58-17820 that a W addition at 1.5% or less is effective in improving creep strength, it does not mention the effect of Nb.